The present invention is related in general to flashing warning lights, and, more particularly, to the provision of a high luminous intensity flashing warning light using superluminescent light emitting diodes for improved visibility and lower power consumption than conventional devices.
Flashing (i.e. intermittently or periodically illuminated) lights have long been used to provide visual warnings, and a considerable body of research has been compiled in the fields of psychology and engineering (and more recently in that hybrid field known as "human factors engineering") concerning human perception of flashing light (i.e. the ability of people to perceive and respond to flashing light). From such research and study, a large number of factors have been identified and suggested as involved in the human perception of and reaction to flashing light, and although much knowledge on the subject is theoretically based and remains to be confirmed, there have presently been suggested certain factors which may be applied to the provision of a flashing warning light for improving the visibility of a flashing light, that is, for making a flashing light visible at a greater distance (i.e. "visibility"), and for enhancing the probability that people will not only perceive (i.e. see) the flashing light but will also react consciously thereto (i.e. "attention-getting").
It is suggested for example from the study of human factors that human visual perception of flashing light appears greatest when the light is flashed at a flash rate or frequency in the range of 3 to 10 flashes per second, with a flash duration of at least 0.05 seconds being recommended. Further, for the flashing of a light to be perceived as discrete flashes, the flash rate or frequency must be below the so-called "flicker-fusion" frequency, that is the frequency above which a flashing light appears as a steady light (i.e. due to the phenomenon of "persistence of vision"), this critical frequency being considered to be approximately 24-30 flashes per second.
For simplicity, hereinafter flash rate or frequency will be described in terms of "flashes-per-second" (fps).
Research has revealed other factors to be pertinent concerning perception of light in general, and flashing light in particular. For example, according to Fechner's Law the sensation of light as produced by the eye varies logarithmically with the intensity of the stimulus. Further, it has been found that at low intensities of light, colors cannot be distinguished ("twilight vision"). It has also been found that at low luminous intensities, before twilight vision, there is a distinct shift of the maximum in the visibility curve towards a shorter wavelength, this phenomenon being known as the Purkinje effect. This shift of the spectral visibility curve tends to make color sensations variable quantities.
With respect to the human perception of flashes of light, a number of psychosensory phenomena of interest have been indicated. It has been suggested for example that a number of different "channels" and "systems" of visual perception are present in humans, and that the perception of flashing light stimulus depends upon the action and interaction of these channels in receiving visual stimuli from the rods and cones of the retina of the eye and in transmitting these stimuli to sensory and motor centers of the brain where the stimuli are "perceived" and responded-to. Further, experimental results have suggested that rod and cone signals elicited by a single flash combined in an excitatory fashion. The results of other experimental work have suggested that light flashes of short duration favor one visual sensory channel, while longer duration flashes favor another visual sensory channel. Still further, with respect to the dynamic range of visual sensory perception in humans, it has been found that short (e.g. 4-msec) test flashes presented on backgrounds appear to have less dynamic range (between above and below threshold) than long (e.g. 300-msec) test flashes. Decreasing the intensity of a short flash has surprisingly little effect until a final small adjustment makes a comparatively brig-ht flash suddenly disappear. Dimming a 300-msec flash is more predictable; the flash grows dimmer until it disappears. It has further been found that, incidentally, short test flashes tend to appear as abrupt bursts, whereas long test flashes are frequently seen as in distinct smudges that drift on, then off.
Luminance discrimination has also been experimentally studied, with regard to what psychosensory mechanisms are involved in discerning or seeing light flashes and in discriminating luminance differences between light flashes, in an attempt to establish psychometric curves for these functions. For example, it has been attempted to demonstrate that there are two discrete detection channels, one for long flashes and one for short flashes. Experiments have shown that different slopes are obtained for psychometric curves measured with short and long flashes. The explanation favored is that the visual system is not homogenous; there are at least two detection channels with inherently different slopes, and it is believed that these can be differentially tapped by varying test flash parameters. Results of some experiments tend to confirm this, and suggest that whereas the long flash detection channel is photometrically subtractive or subadditive, the short flash detection channel is photometrically additive and has a much steeper psychometric function slope than the long flash detection channel. It is further suggested that the psychometric function slopes of the different visual sensory channels vary differently as a function of wavelength, and it has been adduced that all three channels of the visual system do not have the same gain but rather differ in spectral sensitivity.
An interesting question concerns the relationship between the light detection and the flicker threshold. When flashes are supplied within a certain interval, they are perceived as being fused and are indistinguishable from continuously supplied light. It has been almost 150 years since it was shown that, under fused circumstances, the mean intensity over time is independent of the actual light-dark ratio. A further question concerns how many extra quanta of light must be added to flashes perceived as fused at the absolute threshold of vision to perceive a flickering light again, or more precisely, in order to see a regular high-frequency flickering light again (since fused light at threshold level is perceived as irregular flickering light). It has been previously shown that the visual perception system's processing of quantal effects at low luminance levels is essentially nonlinear. Flicker can be detected either by the "on response" or by the "off response" of the visual system to a flash of light. In the case of the on response, extra light quanta have to be supplied so that the threshold set by the adaptational state induced by the previous stimuli is exceeded. A larger interval between flashes leads to a lowering of the adaptational state (because of a decrease of the flux) and thus to lower thresholds. In the case of the off response, the excitation state has to decrease by a certain amount in order to exceed the decrement threshold. If flashes last long enough for a stable adaptation level to be reached, then the threshold no longer depends on the actual flash duration. Experimental results have shown that after 100 msec this stable level can be reached and maintained by a constant intensity in the flashes.
With regard to critical fusion frequency as a function of mean intensity at low luminance levels, it is has been suggested that the critical fusion frequency increases from 6 to 25 Hz with increasing stimulus size. It has also been found that, at higher luminance levels, brief flashes need a longer interval to elicit flicker perception than do long-lasting flashes.
Behavioral studies have shown that when attention is directed towards a point in space, stimuli occurring at or near that location receive facilitated processing. This voluntary control over the spatial focus of attention, which can occur even while the eyes remain stationary, has been described metaphorically as an attentional "spotlight". Some authors have suggested that the attentional spotlight has discrete boundaries and that stimuli falling within this "zone of facilitation" show an "all-or-none" enhancement of processing. Other research suggested that the spotlight is probably flexible, changing its size as a function of task demands.
Recent work, however, has indicated that the spatial distribution of attention in many cases takes the form of a "gradient", such that the falloff of enhanced processing in regions surrounding an attended location is gradual rather than all-or-none. The bulk of evidence supporting the gradient concept has come from studies of simple reaction time (RT) in humans; in general, RT was prolonged as a function of target distance from the attended locus. However, a simple RT measurement does not readily distinguish between a facilitation of sensory processing and the biasing of response and decision processes in this type of task. Thus, gradient effects in RT may have been due to response delays resulting from higher decision criteria for events occurring at a distance from the attended location. An additional confounding factor is that the RT method may cause the subject's attention to be partially diverted and divided rather than strictly focused on the to-be-attended stimulus.
Event-related brain potentials (ERPs) have been used in an attempt to evaluate the spatial distribution of visual attention to possibly provide a partial solution to problems in prior attempts. It has been well established in the study of visual-spatial attention in humans that stimuli at an attended location elicit higher-amplitude ERP components between 80 and 250 msec post-stimulus. There is good reason to believe that these enhanced ERP amplitudes reflect a facilitation of early visual processing in the sensory pathways (i.e. channels). The ERP methodology employed in a particular case examined how human observers distribute their sensory processing capacity among the various elements of a visual display during fully focused attention, to investigate whether a spotlight or gradient effect could be detected. Stimulus in the left, right and midline visual fields were used. Attentional gradients were seen as progressive decrements in amplitudes of the ERPs to the lateral stimuli when attention was directed in turn to evoking stimulus, to the midline, and to the opposite-side stimuli.
In summary, it may be concluded that simple flashes of light elicit a whole range of complex responses from the visual system relating to retinal potentials, subcortical potentials, primary-visual-cortex and associated area potentials, and generalized non-specific responses of the cortex.
Various different types of flashing lights have been known to be used for providing visual alert or warning lights, and have employed incandescent lamps, rare gas discharge lamps and, more recently, light emitting diodes as an illumination means, with some associated control circuitry. However, each of these previous types of illumination means has its disadvantages. Further, the design and operation of such previous types of flashing lights did not take into account the various factors such as flash rates and durations for optimizing the psychosensory perception of flashing light. Still further, the previous flashing light devices could not provide effective light output with low power consumption (i.e. efficiency) at desirable high flash rates, or could not do so without severly sacrificing device power consumption and reliability of the light source, and thus could not provide reliable low power operation and were thus not suitable for use in portable lightweight battery powered equipment.
For example, while incandescent light sources have commonly been used in flashing warning lights, there is the problem that, typically, incandescent light sources are not able to come to full brightness and to then cool off to extinction (i.e. turn on and off) within the higher optimum flash rate frequencies for attracting attention; the flashing character of tungsten-filament lamps is typically degraded significantly above flash rates of 9 fps. Furthermore, because of the inherent thermal inertia of incandescent light sources (once turned sufficiently on to emit light, there is a significant delay in extinction to the off state) as shown in FIG. 1, such light sources cannot provide flashes of relatively short duration, nor can such light sources provide adequate on-off contrast when operated at higher flash rates. As a consequence, incandescent light sources are not suitable for use as warning lights at those flash rates and flash duration periods to which human visual perception is most sensitive but are constrained to use at lower frequencies and longer flash periods.
Still further, incandescent lamps are inefficient due to their emission of considerable energy at wavelengths outside the visual spectrum, and suffer inherent increased power loss, thermal inertia and filament degradation when operated at higher intensity and/or flash rates. An incandescent flashing light with adequate intensity for outdoor use usually requires larger size batteries to compensate for the excessive power loss in the form of heat, thus rendering it impractical for applications requiring reasonably small size and light weight necessary for portability. Durability of incandescent flashing lights is also degraded due to the thermal stress on the filament and mechanical shocks received by the filament.
Rare gas discharge lamps (e.g. Xenon, Argon flash tube lamps and strobes), while- capable of operation at higher flash rates are, however, limited to extremely short flash durations which cannot be lengthened. Thus, such light sources are incapable of longer flash duty cycle operation. Furthermore, rare gas discharge lamps are relatively expensive and must necessarily be energized with high voltages and currents, and thus flashing warning lights of this type require complex charging and discharging circuits and consume considerable power. Furthermore, a large amount of energy is required to produce the flashing action of a rare-gas lamp; it tends to deplete ordinary batteries quickly if flashed at an optimal frequency of 3 to 12 Hz continuously such as that required by a warning light. Therefore, rare-gas discharge lights for extended flashing time are only feasible where a large power source is available, such as the utility power, or a power generator, but not in a portable application. Furthermore, being glass-encased, gas discharge flash tubes are susceptible to mechanical shock damage and to gas leakage rendering them inoperative.
Ordinary light emitting diodes (LEDs) are relatively durable mechanically and electrically (as long as their current supply is properly limited) and most readily lend themselves to low voltage-low current operation and electronic control for both flash rate frequency and duration. However such ordinary LEDs as have previously been used as light sources in flashing warning lights were of insufficiently low light intensity output. Hence the use of such low luminosity light emitting sources in visual warning devices has been of limited effectiveness, being restricted to subdued light environments such as for indoor activities, or where the ambient or background light level is quite low so that sufficient contrast can be obtained with the relatively dim illumination intensity of ordinary LEDs to render them visible against a background. Thus, ordinary LED flashers have only found wide application in toys, jewelry and other devices where visibility requirements are not critical. Examples of such prior devices are found disclosed in U.S. Pat. Nos. 3,786,246 and 3,812,614 (flying disc type toys); U.S. Pat. No. 4,308,572 (clothing ornament); U.S. Pat. No. 4,170,036 (jewelry); and U.S. Pat. No. 4,383,244 (skate wheel).
In order to be both effective and practical, a portable warning light should satisfy at least the following requirements:
1. Adequate visibility, and in an attention-getting manner. This involves considerations of various factors such as: the luminous intensity as well as the on-off contrast ratio of the light source; flash rate/frequency; and flash duration/period.
2. Controllability. This involves the relative ease of controlling the light source for effective flash rate frequency and flash duration.
3. Extended operating battery life. This is a critical factor and requires balancing the interdependent factors of the power available, the light output intensity, and the permissible weight of the device.
4. Durable. This requirement concerns the reliability of the device.
5. Light weight and small size. This requirement constrains the use of large and/or heavy batteries and thus affects the available power, limiting permissible power consumption in order to achieve adequate operating life.
6. Cost. This is often of paramount concern since complex devices not only adversely affect economy in manufacture, but also the applicability of such devices to use by consumers.
Unfortunately, although numerous prior flashing light devices are known, these prior devices have failed to meet or satisfy all of the above-noted requirements for use in a portable flashing warning light.
Portable warning flashers have wide usefulness, one particularly useful application of a portable flashing warning light being as a bicycle or jogger warning signal flasher for alerting vehicular traffic to the presence of bicycles and joggers. Bicycles are frequently ridden on or alongside heavily traveled motor vehicle thoroughfares. Similarly, joggers often run alongside roads and bicycle and jogging paths are often established alongside roads and highways, and may cross roads frequently. In metropolitan areas where traffic is heavy and fast moving during morning and evening rush hours, joggers, pedestrians and bicycle riders are frequently found on and alongside streets, roads and highways during periods of heavy traffic since at these times children may be making their way to and from school, people may be commuting to and from work on bicycles, and joggers often prefer to run during the morning and early evening hours due to cooler conditions at those times. Concomitant with the presence of pedestrians, joggers and bicyclists along vehicular roadways during the early morning and early evening is the fact that, especially during the darker winter months, the risng or setting sun is relatively low on the horizon at such times so that natural outdoor ambient light levels may be low while still not being so dark as to require vehicle drivers to have their headlights on, making it difficult for drivers of motor vehicles to actually discern bicyclists, pedestrians and joggers until they are quite close. Similarly, at dawn and twilight, vision is difficult because of low natural light levels and because of the eyes' difficulty in adapting quickly to the changing from dark to light and vice versa, along with the above-noted shifting of the eyes' spectral response at such times. Such factors, combined with the concentration demands placed upon drivers during heavy traffic conditions, make it difficult for drivers to see bicyclists, joggers and pedestrians along roadways during those times when it is most likely that they will be present, and thus there is a great need for a means of effectively alerting drivers to the presence of bicyclists, joggers and pedestrians along roadways during darkness and semi-darkness.
To be effective, such an alert device should attract a driver's attention at as great a distance as possible from the bicyclist, jogger, etc., given the line of sight situation, so that the driver will have adequate notice and may take appropriate precaution while still approaching and before arriving upon the bicyclist or jogger. This requires first and foremost that the alert signal attract the driver's attention, i.e. be visible, perceptible and noticeable so that the driver will be made aware thereby of the bicyclist's or jogger's presence as early as possible.
Various prior safety flasher light devices have been proposed along these lines. Exemplary of such prior devices are those safety lights disclosed in U.S. Pat. Nos. 4,423,473 and 4,451,871. In these devices, a penlight battery power supply is coupled to an ordinary LED mounted within a lensed refelector housing by means of a position sensitive mercury switch, so that, when the device is worn or carried, the position sensitive switch will on account of sensing the wearer's movements connect power to the LED to cause intermittent bursts of light to be emitted thereby. It is described that because power is supplied only intermittently to the LED, the light source LED may be operated from a battery source which provides current to the light source LED in excess of its maximum current rating to provide light of greater intensity than is normally producible from such light source (i.e. an ordinary low luminosity LED). Such a device however does not produce flashes at any particular effective flash rate (apart from being responsive to the rhythm of the wearer's motions sensed thereby), nor of any particular flash duration and thus is not optimal for attracting attention, nor reliable since it regularly will subject the LED to an overcurrent condition which, while perhaps brief, risks damaging the LED junction nevertheless should the mercury switch connection remain on for too long. Another motion-switched intermittently flashed safety light device is disclosed from U.S. Pat. No. 4,535,392.
In U.S. Pat. No. 4,523,258 there is disclosed a safety belt with flashing LEDs for joggers in which an array of LEDs arranged along a reflective belt are connected to a battery-powered oscillator circuit including two separate oscillators, one oscillating at &lt;1 to 5 Hz, and another oscillating at 3 Hz. Sets or subsets of the LEDs in the array are alternately driven by connecting these oscillators to opposite nodes of the array, such that one oscillator forwardly biases the LEDs and the other oscillator reversely biases the LEDs, such that LEDs of different sets are driven only when forwardly biased and thus flash alternately. However, such a device is only suitable for low ambient light conditions.
In U S. Pat. No. 4,819,135 there is shown a bicycle lighting device in which strings of LEDs are mounted along the frame tubes of a bicycle and flashed in sequence to provide a broadside flashing triangular slow moving vehicle signal to motorists. U.S. Pat. No. 4,763,230 shows a string of LEDs adapted to be secured to the spokes of a bicycle wheel.
Other portable safety flasher lights are known from U.S. Pat. Nos. 3,153,745; 3,840,853; and 4,323,879. Flashing or blinking signal light devices for bicycles and other vehicles are also known from U.S. Pat. Nos. 2,661,406; 3,764,976; 3,916,377; 3,974,369; 3,987,409; 4,019,171; 4,388,559; 4,550,305; 4,598,339; and 4,692,736.
Devices using flashing LEDs are also known from U.S. Pat. Nos. 3,737,722; 4,228,484; and 4,228,485. U.S. Pat. No. 4,271,408 discloses an array of LEDs mounted on a reflectorized substrate to form a colored light source, for use in signs. U.S. Pat. No. 4,654,629 discloses a vehicle marker light for end-of-train equipment use having arrays of LEDs which are driven to be flashed at prescribed or different flash repetition rates at night or during other low visibility conditions.
However, none of the known devices satisfactorily meet the myriad requirements for an effective portable safety warning flasher of high attention-getting visibility at low power consumption and light weight with low cost and high reliability, and thus there has remained a need for a device which can satisfy these requirements.